The Acqua of the Back Arc is a major story arc of Toaru Majutsu no Index. Part of the overarching God's Right Seat Arc storyline, it deals primarily with the attempts of the Amakusa Christian Church's, and later, Kanzaki Kaori, at defeating Acqua of the Back before he completes his attempt in destroying, in what he believes to be, the center of the chaos spreading throughout the world, Kamijou Touma and his Imagine Breaker. It features a second infiltration of Academy City by a God's Right Seat member, the first arc to feature a Saint vs. Saint battle, the first physical appearance of God’s Right Seat leader, Fiamma of the Right, and the fate of Vento of the Front after her defeat in the invasion of Academy City.